This application claims priority under 35 U.S.C. xc2xa7 119 of German Patent Application 00128576.6, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to a gas turbine blade, having a blade aerofoil and a platform region adjacent to the blade aerofoil and bounding a hot gas duct of a gas turbine in which the gas turbine blade may be installed. The present invention also generally relates to a gas turbine with such a gas turbine blade.
2. Background of the Invention
A gas turbine blade is apparent from DE 26 28 807 A. The gas turbine blade is aligned along a blade axis and has a blade aerofoil and a platform region along the blade axis. In the platform region, a platform extends radially outward from the blade aerofoil transverse to the blade axis. Such a platform forms a part of a flow duct for a working fluid, which flows through a gas turbine in which the turbine blade is installed. In a gas turbine, very high temperatures occur in this flow duct. In consequence, the surface of the platform exposed to the hot gas is subject to severe thermal effects. This demands cooling of the platform.
In order to cool the platform, a perforated wall element is arranged in front of the side of the platform facing away from the hot gas. Cooling air passes via the holes in the wall element and impinges on the side of the platform facing away from the hot gas. In a gas turbine, cooling air for the components to be cooled is generally tapped off from a compressor, which generates compressed air for the combustion in the gas turbine. The air quantity which can be supplied to the combustion process is reduced because cooling air is tapped off. This reduces the efficiency of the gas turbine. Efforts are correspondingly made to keep the cooling air consumption in a gas turbine as low as possible.
WO 00/57032 A1 reveals a guide vane for a gas turbine in which the platform is embodied as a separate component for simplification of the covering technology in a casting process. This separate platform component may also include a ceramic material.
U.S. Pat. No. 5,269,651 shows a ceramic guide vane ring which is movably held at its inside by compression of a clamping element. In this arrangement, the inner ring is subdivided into a plurality of piston-ring type elements. Compensation may be provided, by this arrangement, for the axial displacement between the outer and inner casings.
In the Patent Abstracts of Japan, Vol. 014, No. 060 (M-0931), 05.02.1990, a gas turbine guide vane is shown which includes a ceramic shell which is supported by a metallic insert. A thermally insulating layer is arranged between the ceramic shell and the metallic insert.
U.S. Pat. No. 3,867,065 shows a fully ceramic rotor blade arrangement for gas turbines. An annular ceramic insulator is arranged on the inner surface of the inner periphery of the rotor blade structure in order to avoid heat transfer and thermal gradients.
An object of the present invention is to provide a gas turbine blade that has a particularly low requirement for cooling air.
A further object of the present invention is to provide a gas turbine with a particularly low requirement for cooling air.
An object directed toward a gas turbine blade is achieved, according to the present invention, by the provision of a gas turbine blade, having a blade aerofoil and a platform region, adjacent to the blade aerofoil and bounding a hot gas duct of a gas turbine in which the gas turbine blade may be installed, the platform region having a metal platform on which a ceramic covering is supported and fastened by way of a mechanical fastening device.
The present invention initiates a completely new way of providing the platform of a gas turbine blade, where platform bounds the hot gas duct, with a mechanically fastened ceramic covering. The metal platform is effectively screened from hot gas flowing through the hot gas duct by the ceramic covering. Correspondingly, the metal platform requires distinctly less cooling. Under certain circumstances, it may even be possible to dispense entirely with cooling of the metal platform. The result of this is a substantially reduced requirement of cooling air, which in turn increases the efficiency of the gas turbine in which the gas turbine blade is installed.
The gas turbine blade of the type proposed may, furthermore, be easily manufactured because it is only necessary to change a conventional gas turbine blade somewhat with respect to its radial dimensions. Thus, the ceramic covering may be positioned flush to the hot gas duct.
In other respects, the gas turbine blade may be conventionally manufactured, in particular by casting. The ceramic covering can be later supported and fastened onto the metal platform by way of the mechanical fastening element. In particular, it is possible to install such gas turbine blade in a blade ring in the gas turbine and, in the process, join the ceramic covering, piece by piece, to each installed gas turbine blade. Therefore, the result is a complete and closed blade ring, which additionally clamps the ceramic coverings from falling out.
The ceramic covering may also be exchanged later in a simple manner, perhaps during routine servicing, by simply supporting it on the metal platform and fastening it by way of the fastening element.
a) The ceramic covering preferably includes two halves. One half is, furthermore, preferentially adjacent to a suction surface of the blade aerofoil and the other half is adjacent to a pressure surface of the blade aerofoil. The application of the ceramic covering is then of particularly simple arrangement because the two halves of the ceramic covering are simply attached around the blade aerofoil.
b) The mechanical fastening device is preferably a spring, which is firmly connected to the gas turbine blade. A sprung fastening of the ceramic covering is therefore achieved by way of the fastening device. This has, in particular, the advantage that any vibrations of the gas turbine blade are transferred in a damped manner to the ceramic covering, which reduces any danger of fracture to the ceramic covering. In addition, the spring preferably engages in a groove of the ceramic covering, which groove extends along a narrow side adjacent to the blade aerofoil.
c) A fixing pedestal is preferably arranged on the metal platform, which pedestal engages in the ceramic covering. By way of such a fixing pedestal, the ceramic covering is fixed, against sliding on the metal platform, additionally to the fastening by way of the fastening element.
d) The gas turbine blade is preferably configured as a guide vane, which has a second platform region which, together with the platform region, encloses the vane aerofoil and is opposite to the platform region. The second platform region has a second metal platform on which a second ceramic covering is supported and is fastened by way of a second mechanical fastening device. A gas turbine guide vane usually has two platform regions. One platform region is adjacent to an engagement arrangement of the gas turbine guide vane by way of which the gas turbine guide vane is engaged in a casing of a gas turbine. The second platform region bounds the hot gas duct opposite to a gas turbine rotor. Both platform regions can be provided with a ceramic covering.
e) The ceramic covering preferably has an integral mat, by way of which the fragments are held as a composite in the event of a fracture of the ceramic covering. Ceramic is substantially more brittle than metal and is subject to the danger of splintering, perhaps on the impingement of a solid body flowing in the hot gas duct. In the case of a fracture of the ceramic covering, fragments could pass into the hot gas duct and damage subsequent turbine blading stages in the hot gas duct. This is prevented by the integral mat of the ceramic covering. In the case of a fracture of the ceramic covering, the fragments are held together by the mat. The mat may, for example, be introduced into the ceramic covering, for example by casting it in during the manufacture of the ceramic covering. The mat may also, however, be joined to the bottom of the ceramic covering.
f) The ceramic covering preferably exhibits mullite. Mullite is a particularly suitable material with particularly suitable properties in terms of thermal resistance and also in terms of resistance to oxidation and corrosion.
g) The ceramic covering preferably has an outer sealing to combat particle separation. The ceramic covering may include a ceramic basic body whose surface tends to release solid body particles. These may have an erosive effect in the subsequent hot gas duct on the gas turbine blading which follows there. A sealing layer combats this release of particles.
The embodiments described in the paragraphs a) to g) can be combined together in any given manner.
According to the present invention, the object directed toward a gas turbine is achieved by the provision of a gas turbine with a gas turbine blade according to one of the embodiments described above.
The advantages for such a gas turbine follow correspondingly from the above statements relating to the advantages of the gas turbine blade.
The gas turbine blade is preferably arranged, in the axial direction of a flow duct of a gas turbine, between two rotor blades, whereby the second ceramic covering extends in the axial direction just so far as not to be rubbed by one of the rotor blades. This reliably prevents the ceramic covering from being damaged by a rub due to the rotor blades respectively adjacent to it and rotating past it.